(i) Field of the Invention
The present invention relates to an optical fiber fixing member for positioning/engaging optical fiber end portions, an optical fiber array in which the optical fiber end portions are positioned/fixed, an optical waveguide module, and a method of measuring dimensional accuracy of the optical fiber fixing member.
(ii) Description of the Related Art
An optical fiber fixing member is used in a member for positioning/fixing light input/output ends of optical fibers with high accuracy. Such an optical fiber fixing member, for example, is disclosed as a substrate for fixing the optical fiber in Japanese Patent Application Laid-open No. 292332/1996 (hereinafter referred to as "the publication"). Fixing grooves for receiving and positioning the optical fibers are formed in a surface of the board by press molding. Each fixing groove has a V-shaped sectional configuration, and its bottom face is pointed or flat. The publication further discloses an optical fiber array in which the optical fibers are contained in the fixing grooves and pressed with a lid.
After the optical fiber fixing member is prepared by grinding processing, press molding, or another method, the optical fiber engaging portions for positioning/engaging optical fiber end portions need to be examined to determine if they are formed within a predetermined accuracy. For example, in a case where a plurality of optical fibers are positioned/engaged and aligned, if the accuracy in the pitch of the optical fiber engaging portions is not within the predetermined range, they cannot be used in an optical fiber array where a low optical connection loss is demanded. Examples of a conventional method of measuring a pitch dimensional accuracy include an optical measuring method in which the accuracy is measured from an image with a measuring microscope or the like, and a stylus type measuring method in which a configuration measuring device using a stylus is employed.
(1) In a case where the pitch dimension of each optical fiber engaging portion is measured from an image with a measuring microscope or the like, for example, a method of measuring a pitch or the like between the centers of bottom faces of adjoining optical fiber engaging portions can be used.
However, if the bottom face of the optical fiber engaging portion is pointed as disclosed in the publication, it is difficult to obtain a contrast of the image for detecting a center position of the bottom face of the optical fiber engaging portion. Moreover, even when the bottom face of the optical fiber engaging portion is flat as disclosed in the publication, it is difficult to distinguish the inclined faces and the flat bottom face constituting the fixing groove by image processing if the bottom face is coarse. Therefore, pitches cannot be automatically measured by image processing with a measuring microscope or the like.
For example, if the bottom face is pointed, inclined faces of the optical fiber engaging portion for supporting a side wall of the optical fiber directly abut on each other. Therefore, it is difficult to automatically bring a boundary line between the inclined faces into focus. At the time of observation with the microscope, reflectances of the two inclined faces relative to a light are so close to each other that the boundary line between the inclined faces is not apparent. Moreover, even if the boundary line can be observed with the naked eye, a processing streak on the inclined face is frequently mistaken for the boundary line in the detection of the boundary line by the image processing.
On the other hand, in a case where the bottom face is flat but coarse, light is irregularly reflected by the bottom face. Therefore, both the bottom face and the inclined face are reflected rather than white, and the boundary between the bottom face and each inclined face provides little contrast. Moreover, when the boundary between the bottom face and each inclined face is constituted by a moderate curved face, the boundary between the bottom face and each inclined face provides little contrast, and it is even more difficult to distinguish the boundary from the image.
As aforementioned, there is a problem in that the center position of the optical fiber engaging portion cannot be easily found when the image exhibits a low contrast between the flat bottom face and the inclined face and when the boundary line between the bottom face and the inclined face is unclear.
(2) In the measuring method in which a stylus type configuration measuring device is used, the surface of the optical fiber engaging portion is traced perpendicularly to a groove direction (an optical axis of the engaged optical fiber), with a stylus having a tip-end radius of about 1 to 30 .mu.m, to first measure an outline sectional configuration of the optical fiber engaging portion. Subsequently, the obtained outline sectional configuration is analyzed with analysis software to obtain accuracy in the pitch and depth of the optical fiber engaging portion.
To accurately measure pitches, however, the squareness of the groove direction of the optical fiber engaging portion (the optical axis direction of the optical fiber, the extending direction of the optical fiber engaging portion) needs to be accurately adjusted to that of the measuring direction (the scanning direction of the stylus).
If the squareness adjustment is not performed accurately, the pitch is measured larger only by .DELTA.P=P((1/cos .DELTA..theta.)-1), in which .DELTA.P represents a pitch error, .DELTA..theta. represents an angle deviated from the right angle, and P represents a true pitch. For example, in a general optical fiber fixing member having eight V-shaped grooves at 250 .mu.m pitches, an accumulated pitch is 250.times.7=1750 .mu.m. Therefore, even if .DELTA..theta. is one degree, the pitch is measured as larger by 0.27 .mu.m. In a fixing member for single-mode optical fibers, since the latest allowable pitch error is .+-.0.5 .mu.m or less, an error of 0.27 .mu.m in the measurement is not acceptable. Contrarily, to set the allowable error to 0.1 .mu.m or less in the measurement, the error of squareness has to be set to 0.64 degrees or less.
As aforementioned, even in the accuracy measurement by the stylus type configuration measuring device, the scanning direction of the stylus needs to be accurately adjusted. Therefore, when the position of the groove bottom face of the optical fiber engaging portion is not easily detected, the adjustment of the stylus scanning direction requires much labor. Moreover, in order to realize automation of scanning direction adjustment, the position of the groove bottom face of the optical fiber engaging portion needs to be detected with high contrast on the observed image.
(3) The problems described above also arise when the optical fibers are engaged in the optical fiber engaging portions to assemble an optical fiber array. For example, when the optical fiber is engaged in the optical fiber engaging portion, the vicinity of the optical fiber engaging portion is observed from above and enlarged with a microscope or the like and, for example, an optical fiber position is finely regulated with a precision stage in such a manner that the optical axis of the optical fiber is positioned in the center of the optical fiber engaging portion. In this case, with the conventional optical fiber fixing member, the center position of the optical fiber engaging portion is not clear. This is especially true when a multi-core optical fiber is engaged, as much labor is necessary. To automate the operation of engaging the optical fiber in the optical fiber engaging portion, the center position of the optical fiber engaging portion needs to be recognized by the image processing. However, since the image processing is inferior in boundary detecting capability to the naked eye, the contrast in the boundary portion needs to be raised.
(4) Furthermore, after the optical fiber array constituted by the optical fiber fixing member, the optical fibers and a lid is assembled, in some cases it needs to be examined whether the optical axis of the optical fiber extends along the center of the optical fiber engaging portion in the input/output end portion of the optical fiber and the vicinity of an end portion extending along the optical fiber. In this case, if either the optical fiber fixing member or the lid is transparent, the vicinity of the optical fiber engaging portion in which the optical fiber is engaged/fixed can be observed by a microscope. However, in the conventional optical fiber array, since the center of the optical fiber engaging portion, as a reference, cannot easily be recognized as aforementioned, much labor is necessary for high-precision examination of positional accuracy.